We suggest that the partial pressure of carbon dioxide in the atmosphere is buffered, over geological time scales, by a negative feedback mechanism in which the rate of weathering of silicate minerals (followed by deposition of carbonate minerals) depends on surface temperature, and surface temperature, in turn, depends on carbon dioxide partial pressure through the greenhouse effect. Although the quantitative details of this mechanism are speculative, it appears able partially to stabilize earth's surface temperature against the steady increase of solar luminosity believed to have occurred since the origin of the solar sys tern.

A simple, idealized model for the rapid escape of a hydrogen thermosphere provides some quantitative estimates for the energy-limited flux of escaping particles. The model assumes that the atmosphere is "tightly bound" by the gravitational field at lower altitudes, that diffusion through the lower atmosphere does not limit the flux, and that the main source of heating is solar euv. Rather low thermospheric temperatures are typical of such escape and a characteristic minimum develops in the temperature profile as the escape flux approaches its maximum possible value. The flux is limited by the amount of euv energy absorbed, which is in turn controlled by the radial extent of the thermosphere. Regardless of the amount of hydrogen in the thermosphere, the low temperatures accompanying rapid escape limit its extent, and thus constrain the flux. Applied to the Earth and Venus, the results suggest that the escape of hydrogen from these planets would have been energy-limited if their primordial atmospheres contained total hydrogen mixing ratios exceeding a few percent. Hydrogen and deuterium may have been lost in bulk, but heavier elements would have remained in the atmosphere. These results place constraints on hypotheses for the origin of the planets and their subsequent evolution.

Carbonate and organic matter deposited during the latest Paleocene thermal maximum is characterized by a remarkable -2.5% excursion in delta 13C that occurred over approximately 10(4) yr and returned to near initial values in an exponential pattern over approximately 2 x 10(5) yr. It has been hypothesized that this excursion signifies transfer of 1.4 to 2.8 x 10(18) g of CH4 from oceanic hydrates to the combined ocean-atmosphere inorganic carbon reservoir. A scenario with 1.12 x 10(18) g of CH4 is numerically simulated here within the framework of the present-day global carbon cycle to test the plausibility of the hypothesis. We find that (1) the delta 13C of the deep ocean, shallow ocean, and atmosphere decreases by -2.3% over 10(4) yr and returns to initial values in an exponential pattern over approximately 2 x 10(5) yr; (2) the depth of the lysocline shoals by up to 400 m over 10(4) yr, and this rise is most pronounced in one ocean region; and (3) global surface temperature increases by approximately 2 degrees C over 10(4) yr and returns to initial values over approximately 2 x 10(6) yr. The first effect is quantitatively consistent with the geologic record; the latter two effects are qualitatively consistent with observations. Thus, significant CH4 release from oceanic hydrates is a plausible explanation for observed carbon cycle perturbations during the thermal maximum. This conclusion is of broad interest because the flux of CH4 invoked during the maximum is of similar magnitude to that released to the atmosphere from present-day anthropogenic CH4 sources.

Detailed investigations of high latitude sequences recently collected by the Ocean Drilling Program (ODP) indicate that periods of rapid climate change often culminated in brief transient climates, with more extreme conditions than subsequent long term climates. Two examples of such events have been identified in the Paleogene; the first in latest Paleocene time in the middle of a warming trend that began several million years earlier: the second in earliest Oligocene time near the end of a Middle Eocene to Late Oligocene global cooling trend. Superimposed on the earlier event was a sudden and extreme warming of both high latitude sea surface and deep ocean waters. Imbedded in the latter transition was an abrupt decline in high latitude temperatures and the brief appearance of a full size continental ice-sheet on Antarctica. In both cases the climate extremes were not stable, lasting for less than a few hundred thousand years, indicating a temporary or transient climate state. Geochemical and sedimentological evidence suggest that both Paleogene climate events were accompanied by reorganizations in ocean circulation, and major perturbations in marine productivity and the global carbon cycle. The Paleocene-Eocene thermal maximum was marked by reduced oceanic turnover and decreases in global delta 13C and in marine productivity, while the Early Oligocene glacial maximum was accompanied by intensification of deep ocean circulation and elevated delta 13C and productivity. It has been suggested that sudden changes in climate and/or ocean circulation might occur as a result of gradual forcing as certain physical thresholds are exceeded. We investigate the possibility that sudden reorganizations in ocean and/or atmosphere circulation during these abrupt transitions generated short-term positive feedbacks that briefly sustained these transient climatic states.

We show that mass fractionation occurs during the course of hydrodynamic escape of gases from the atmosphere of an inner planet. Light gases escape more readily than heavy gases. The resultant fractionation as a function of mass yields a linear or concave downward plot in a graph of logarithm of remaining inventory against atomic mass. An episode of hydrodynamic escape early in the history of Mars could have resulted in the mass-dependent depletion of the noble gases observed in the Martian atmosphere, if Mars was initially hydrogen rich. Similarly, a hydrodynamic escape episode early in Earth's history could have yielded a mass-dependent fractionation of the xenon isotopes. The required hydrodynamic escape fluxes and total amounts of hydrogen lost from the planets in these episodes are large, but not impossibly so. The theory of the mass fractionation process is simple, but more work will be needed to put together an internally consistent scenario that reconciles a range of data from different planets.